Trimethylsilanol (TMS) is a low molecular weight / low boiling point silicon-containing, airborne contaminant that has
received increased interest over the past few years as an important cause for contamination of optical surfaces in
TMS is not captured well by carbon-based filters, and hexamethyldisiloxane (HMDSO), even though captured well, can
be converted to TMS when using acidic filter media commonly used for ammonia removal. TMS and HMDSO co-exist
in a chemical equilibrium, which is affected by the acidity and moisture of their environment.
This publication shows that HMDSO is converted to TMS by acidic media at concentrations typically found in
cleanroom environments. This is contrary to published results that show a re-combination of TMS to HMDSO on acid
We also demonstrate that, based on its conversion to TMS, HMDSO is not a suitable test compound for hybrid chemical
filter performance, as the apparent lifetime/capacity of the filter can be substantially skewed towards larger numbers when conversion to TMS is involved. We show lifetime test results with toluene and HMDSO on acidic and non-acidic filter media.
Appropriately designed, asymmetric hybrid chemical filters significantly minimize or eliminate the conversion of
HMDSO to TMS, thereby reducing the risk to scanner optical elements. Similarly, such filters can also prevent or reduce acid-sensitive reactions of other AMC when passing through filter systems.